The high pressure die casting (HPDC) process is the most popular casting process for aluminum alloys and is prevalent in the casting of magnesium alloys. 70% of all aluminum cast is accomplished through the HPDC process. Salt cores may also be used in other types of casting processes. The HPDC process is somewhat limited because design features have to be compatible with the opening and closing of dies in the horizontal direction.
Salt cores are used in the HPDC process to assist in the economical production of complex parts. The most widely used expendable, high integrity core used in HPDC is a fused salt core. It generally can withstand the dynamic and static forces of the die casting process, the temperature of the metal and yet is easily removed by dissolving the core with water. The wash out of the salt core is fast and easy. For example, with HPDC engine blocks, two blocks with salt cores may be placed in a water spray cabinet and washed out in the time it takes to make another engine block. The used salt can be recycled or it can be allowed to flow down the drain because it is food quality grade and not toxic.
It is the general practice to make fused salt cores by a die casting process where the molten salt mixture is poured into a die having the shape of the salt core desired to be used in the subsequent HPDC process. Still, salt cores can pick up moisture if stored for a week. Thus, salt cores are made in the casting plant that uses them with “on demand” processing. The 100% salt concentration cores (unlike salt at 3.5% concentrations in water) are not corrosive to any steel part of the die casting machine. Spills can easily be cleaned up and anything the molten salt solidifies on or spills onto while casting the cores may be ignored until it presents a physical barrier to activity.
Fused salt cores are made typically and traditionally from a mechanical mixture of NaCl and Na2CO3. This mechanical mixture is molten and introduced to a mold to create the desired shape of the salt core. The salt core thus produced is used in the HPDC process for casting a metal part. The salt core is spaced from the walls of the die to provide a die cavity, and a molten metal, such as an aluminum alloy, having a melting point less than the melting point of the salt core, is introduced into the die cavity and on solidifying provides a cast metal part. The cast part is then removed from the die and the salt core is removed from the cast part by washing the part in a solvent, such as water, that will dissolve the salt core.
The mixture of NaCl and Na2CO3 is advantageous for casting aluminum alloys because the eutectic temperature of the mixture is 635° C., 83° C. higher than the eutectic temperature of aluminum silicon (AlSi) alloys at 577° C. However, as shown in FIG. 1, NaCl and Na2CO3 exhibit no solubility in one another. Since NaCl and Na2CO3 are each independently brittle, the resulting eutectic which is a mechanical mixture of pure NaCl and pure Na2CO3 is likewise brittle, yielding the high possibility of shrinkage of the two different salts resulting in fissures in the salt core that ultimately affects the quality of the final cast part.
The brittleness of these pure salts is readily interpreted in terms of the packing arrangement of the ions. Deformation of the crystal, that is, sliding of one plane of atoms past another, for a distance of about one ion diameter, bring ions of like charge into contact. Their strong mutual repulsion breaks the crystal. The break occurs between the planes of atoms and leads to new crystal faces.
Thus, certain limitations exist with traditional salt cores, particularly when the shrinkage of the salt upon cooling is significantly greater than the shrinkage of the metal die. This shrinkage leads to cracking of the core that increases the rejection rate of the HDPC cast metal object. Accordingly, developing salt cores that have reduced shrinkage values will result in HPDC castings that have significantly less flaws in the cavities formed by the salt cores.